5 files changed, 97 insertions, 0 deletions
diff --git a/syn-mid/examples/const_fn/Cargo.toml b/syn-mid/examples/const_fn/Cargo.toml
new file mode 100644
index 0000000..d823e76
--- /dev/null
+++ b/syn-mid/examples/const_fn/Cargo.toml
@@ -0,0 +1,16 @@
+[package]
+name = "const_fn"
+version = "0.0.0"
+authors = ["Taiki Endo <te316e89@gmail.com>"]
+edition = "2018"
+publish = false
+
+[lib]
+proc-macro = true
+path = "lib.rs"
+
+[dependencies]
+proc-macro2 = "1.0"
+quote = "1.0"
+syn = "1.0"
+syn-mid = { version = "0.4", path = "../..", features = ["clone-impls"] }
diff --git a/syn-mid/examples/const_fn/lib.rs b/syn-mid/examples/const_fn/lib.rs
new file mode 100644
index 0000000..29255f2
--- /dev/null
+++ b/syn-mid/examples/const_fn/lib.rs
@@ -0,0 +1,31 @@
+#![warn(rust_2018_idioms)]
+
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use proc_macro2::TokenStream as TokenStream2;
+use quote::quote;
+use syn_mid::ItemFn;
+
+/// An attribute for easy generation of a const function with conditional compilations.
+#[proc_macro_attribute]
+pub fn const_fn(args: TokenStream, function: TokenStream) -> TokenStream {
+    assert!(!args.is_empty(), "requires an argument");
+
+    let mut function = syn::parse_macro_input!(function as ItemFn);
+    let mut const_function = function.clone();
+
+    if function.constness.is_some() {
+        function.constness = None;
+    } else {
+        const_function.constness = Some(Default::default());
+    }
+
+    let args = TokenStream2::from(args);
+    TokenStream::from(quote! {
+        #[cfg(not(#args))]
+        #function
+        #[cfg(#args)]
+        #const_function
+    })
+}
diff --git a/syn-mid/examples/const_fn_test/Cargo.toml b/syn-mid/examples/const_fn_test/Cargo.toml
new file mode 100644
index 0000000..b3e2807
--- /dev/null
+++ b/syn-mid/examples/const_fn_test/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "const_fn_test"
+version = "0.0.0"
+authors = ["Taiki Endo <te316e89@gmail.com>"]
+edition = "2018"
+publish = false
+
+[dependencies]
+const_fn = { path = "../const_fn" }
diff --git a/syn-mid/examples/const_fn_test/build.rs b/syn-mid/examples/const_fn_test/build.rs
new file mode 100644
index 0000000..bebf234
--- /dev/null
+++ b/syn-mid/examples/const_fn_test/build.rs
@@ -0,0 +1,16 @@
+use std::{env, process::Command};
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+
+    if is_nightly() {
+        println!("cargo:rustc-cfg=nightly");
+    }
+}
+
+fn is_nightly() -> bool {
+    env::var_os("RUSTC")
+        .and_then(|rustc| Command::new(rustc).arg("--version").output().ok())
+        .and_then(|output| String::from_utf8(output.stdout).ok())
+        .map_or(false, |version| version.contains("nightly"))
+}
diff --git a/syn-mid/examples/const_fn_test/tests/test.rs b/syn-mid/examples/const_fn_test/tests/test.rs
new file mode 100644
index 0000000..1b2c742
--- /dev/null
+++ b/syn-mid/examples/const_fn_test/tests/test.rs
@@ -0,0 +1,25 @@
+#![cfg_attr(nightly, feature(const_fn, const_vec_new))]
+#![warn(rust_2018_idioms)]
+#![allow(dead_code)]
+
+use const_fn::const_fn;
+
+#[const_fn(nightly)]
+fn const_vec_new<T>() -> Vec<T> {
+    let vec = Vec::new();
+    vec
+}
+
+#[test]
+fn test_stable() {
+    assert_eq!(const_vec_new::<u8>(), Vec::new());
+}
+
+#[cfg(nightly)]
+const CONST_UNSTABLE: Vec<u8> = const_vec_new();
+
+#[cfg(nightly)]
+#[test]
+fn test_unstable() {
+    assert_eq!(CONST_UNSTABLE, Vec::new());
+}